Wafer bonding techniques may be utilized to form semiconductor-on-insulator (SOI) substrates. Such techniques typically involve steps to form an electrically insulating layer on a first wafer and then bond the first wafer to a second wafer with the electrically insulating layer disposed therebetween and acting as an adhesion layer. One of the wafers may then be polished using conventional techniques to form a semiconductor active layer that is electrically isolated from the other wafer by the electrically insulating layer. An example of such a bonding technique is described in U.S. Pat. No. 5,753,562 to Kim, entitled "Methods of Forming Semiconductor Devices in Substrates Having Inverted-Trench Isolation Regions Therein", assigned to the present assignee, the disclosure of which is hereby incorporated herein by reference.
Recently, BPSG (borophosphosilicate glass) layers have been considered as adhesion material layers. When such layers are used in an SOI process, annealing steps may be performed at temperatures less than 850.degree. C. to increase bond strength. However, during such annealing steps to increase bond strength, impurities and contaminants can out-diffuse from the BPSG layer and into the first and second wafers where they may adversely influence the electrical characteristics of devices subsequently formed therein.
Referring now to FIG. 1, a cross-sectional view of a conventional semiconductor substrate (e.g., SOI substrate) will be described. In particular, the substrate of FIG. 1 includes a processing wafer 10 (wherein a variety of active semiconductor devices are to be formed), a handling wafer 12, a BPSG layer 14 and a silicon nitride layer 13. As will be understood by those skilled in the art, the BPSG layer 14 acts as an adhesion layer and the silicon nitride layer 13 acts as a diffusion barrier layer to inhibit direct transfer of boron and phosphorus impurities from the BPSG layer 14 to the processing wafer 10 during subsequent thermal treatment steps (e.g., annealing).
In order to increase the degree of bonding (i.e., bond strength) between the processing and handling wafers 10 and 12, an annealing step is typically carried out at temperature of about 950.degree. C. However, during such an annealing step, phosphorous and boron impurities may diffuse out from the ends of the BPSG layer 14 (as illustrated by the arrows in FIG. 1) and contaminate the bonded wafers 10 and 12. When these contaminated wafers are then loaded into new manufacturing apparatus, the manufacturing apparatus and handling devices may also become contaminated with boron and phosphorous impurities.
To solve these contamination problems, attempts have been made to use adhesion materials other than BPSG. For example, undoped silicate glass (USG) has been considered as an alternative to BPSG. Unfortunately, USG typically does not provide a sufficiently strong bond between wafers. Attempts to increase the bond strength of USG have included increasing the annealing temperature, but such increases in annealing temperature can produce parasitic voids in the bond interface. Thus, notwithstanding the above described methods of bonding wafers and forming SOI substrates, there continues to be a need for improved wafer bonding methods which are more suitable for forming SOI substrates.